gemmation from a parent organism, passes gradually from a state in whicli it is an indistinguishable part of the parent organism, to a state in which it is a separate organism of like structure with the parent. At what stage does it become an individual? And if its individuality be conceded only when it completely separates from the parent, must we deny individuality to all organisms thus produced, w^hich permanently retain their connexions with their parents? Or again, what must we say of the Hectocotylus, which is an arm of the Cuttle-fish that undergoes a special development, and then detaching itself, lives independently for a considerable period? And what must we say of that larval Echinus, which is left to move about awhile after being robbed of its viscera by the young Echinus developed within it?
To answer such questions, we must revert to the definition of Life. The distinction between individual in its biological sense, and individual in its more general sense, must consist in the manifestation of Life, properly so called. Life we have seen to be, '^ the definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences." Hence, a biological individual is any concrete whole having a structure which enables it, when placed in appropriate conditions, to continuously adjust its internal relations to external relations, so as to maintain the equilibrium of its functions. In pursuance of this conception, we must consider as individuals, all those wholly or partially independent organized masses, which arise by multicentral and multiaxial development that is either continuous or discontinuous (§ 50). We must accord the title to each separate aphis, each polype of a polypedom, each bud or shoot of a flowering plant, whether it detaches itself as a bulbil or remains attached as a branch.
By thus interpreting the facts, we do not, indeed, avoid all anomalies. While, among flowering plants, the power of independent growth and development, is usually possessed only by shoots or axes; yet, in some cases, as in that of the Begonialeaf awhile since mentioned, the appendage of an axis, or even a small fragment of such appendage, is capable of initiating and carrying on the functions of life; and in other cases, as shown by M. K^audin in the Drosera intermedia^ yo^^g plants are occasionally developed from the surfaces of leaves, while still connected with the parent plant. Nor among forms like the compound Hydrozoa, does the definition enable us to decide where the line is to be drawn between the individuality of the group and the individualities of the members — merging into each other, as these do, in different degrees. But, as before said, such difficulties must necessarily present themselves, if organic forms have arisen by insensible gradations. We must be content with a course which commits us to the smallest number of incongruities; and this course is, to consider as an individual, any centre or axis that is capable of independently carrying on that continuous adjustment of inner to outer relations which constitutes Life.
CIIAPTEE YII.
CIIAPTEE YII.
§ 75. Having concluded what constitutes an individual, we are in a position to deal with the multiplication of individuals. For this, the title Genesis is here chosen, as being the most comprehensive title — the least specialized in its meaning. By some biologists. Generation has been used to signify one method of multiplication, and Reproduction to signify another method; and each of these words has been thus rendered in some degree unfit to signify multiplication in general.
Here the reader is indirectly introduced to the fact, that the production of new organisms is carried on in fundamentally unlike ways. Up to quite recent times, it was believed, even by naturalists, that all the various processes of multiplication observable in different kinds of organisms, have one essential character in common: it was supposed that in every species, the successive generations are alike. It has now been proved, hoAvever, that in plants, and in numerous animals, the successive generations are not alike; that from one generation there proceeds another whose members differ more or less in structure from their parents; that these produce others like themselves, or like their parents, or like neither; but that eventually, the original form re-appears. Instead of there being, as in the cases most familiar to us, a constant recurrence of the same form, there is a cyclical recurrence of the same form. These two distinct processes of multiplication, may be aptlj^ termed homogenesis and heterogenesisJ* Under these heads let us consider them more closely.
The kind of genesis, once supposed to be universal, in which the successive generations are alike, is always sexual genesis; or, as it has been otherwise called — gamogenesis. In every species of organism which multiplies by homogenesis, each generation consists of males and females; and from the fertilized germs they produce, the next generation of similar males and females arises. This method of propagation is further distinguished by the peculiarity, that each fertilized germ gives rise to but one individual — the product of development is always organized round one axis, and not round several axes. Between the different kinds of homogenesis, the most marked contrast, and the only one which need here detain us, is that between the oviparous and the viviparous. The oviparous kind is that in which the fertilized germ is detached from the parent, before it has undergone any considerable development. The viviparous kind is that in which development is considerably advanced, or almost completed, before final detachment takes place. This distinction is, however, not a sharply- defined one: there are transitions between the oviparous and the viviparous processes. In ovo- viviparous genesis, there is an internal incubation; and though the young are in this case finally detached from the parent in the shape of eggs, they do not leave the parent's body until after they have assumed something like the parental form. Looking around, we find that homogenesis is universal among the Verteh'cita: there is no known vertebrate animal but what arises from a fertilized germ, and unites into its single individuality the whole products of this fertilized germ. In * Unfortunately the word Jieterogemsis, has been already used as a synonyme for " spontaneous generation." Save by those few who believe in " spontaneous generation," however, little objection will be felt to using the word in a sense that seems much more appropriate.
the mammals or highest Vcrtchrata, this homogcnesis is in every case viviparous; in. birds it is uniformly oviparous; and in reptiles and fishes, it is always essentially oviparous, though there are cases, of the kind above referred to, in which viviparity is simulated. Passing to the Invertebrata, we find oviparous homogenesis universal among the Arachnida (except the Scorpions, which are ovo-viviparous); universal among the higher Crustacea, but not among the lower; extremely general, though not universal, among Insects; and universal among the higher Mollusca, though not among the lower. Along with extreme inferiority among animals, we find homogenesis to be the exception rather than the rule; and in the vegetal kingdom, there appear to be no cases, save those of a few aberrant parasites like the Raffle siacece J in which the centre or axis which arises from a fertilized germ, becomes the immediate producer of fertilized germs.
germs.
Where propagation is carried on by heterogenesis, or is characterized by unlikeness of the successive generations, there is always asexual genesis with occasionally-recurring sexual genesis; in other words — agamogenesis interrupted more or less frequently by gamogenesis. If we set out with a generation of perfect males and females; then, from their ova or seeds, there arise individuals that are neither males nor females, but that produce the next generation from buds. By this method of multiplication, many individuals originate from a single fertilized germ: the product of development is organized round more than one centre or axis. The simplest form of heterogenesis is that seen in uniaxial plants. If, as we find ourselves obliged to do, we regard each separate shoot or axis of growth, as a distinct individual; then, in uniaxial plants, the successive individuals are not represented by the series A, A, A, A, &c., like those resulting from homogenesis; but they are represented by the series A, B, A, B, A, B, &c. For in plants which were before classed as uniaxial (§ 50), and which may 212 THE INDUCl'IONS OF BIOLOGY.
be conyeniently so distinguished from other plants, the axis which shoots up from the seed, and substantially constitutes the plant, does not itself flower and bear seed; but gives lateral origin to flowering, or seed -bearing, axes. Though in uniaxial plants, the fructifying apparatus appears to be at the end of the primary, vertical axis; yet dissection shows that, morphologically considered, each fructifying axis is usually an offspring from the primary axis. There arises from the seed, a sexless individual, from which spring by gemmation, individuals having reproductive organs; and from these there result fertilized germs or seeds, that give rise to sexless individuals. That is to say, gamogenesis and agamogenesis alternate: the peculiarity being, that the sexual individuals arise from the sexless ones by continuous development. The Salpce show us an allied form of heterogenesis in the animal kingdom. Individuals developed from fertilized ova, instead of themselves producing fertilized ova, produce, by gemmation, strings of individuals; from which fertilized ova again originate. In multiaxial plants, we have a succession of generations represented by the series A, B, B, B, &c.. A, B, B, B, &c. Supposing A to be a flowering axis, or sexual individual; then, from any fertilized germ it casts off", there grows up a sexless individual, B; from this there bud-out other sexless individuals, B; and so on for generations more or less numerous; until at length, from some of these sexless individuals, there bud-out seed-bearing individuals of the original form A. Branched herbs, shrubs, and trees, exhibit this form of heterogenesis: the successive generations of sexless individuals thus produced, being in most cases continuously developed, or aggregated into a compound individual; but being in some cases discontinuously developed. Among animals, a kind of heterogenesis represented by the same succession of letters, occurs in such compound polypes as the Scrtularia; and in those of the Hi/drozoa which assume alternately the polypoid form, and the form of the Medusa: the chief differences presented by these groups, arising from the fact that the successive generations of sexless individuals produced by budding, are in some cases continuously developed, and in others discontinuously developed; and from the fact that, in some cases, the sexual individuals give off their fertilized germs while still growing on the parent-polypedom, but in other cases, not until after leaving the parent-polypedom and undergoing further development. Where, as in all the foregoing kinds of agamogenesis, the new individuals bud-out, not from any specialized reproductive organs, but from unspecialized parts of the parent; the process has been named, by Prof. Owen, metagenesis. In most instances, the individuals thus produced, grow from the outsides of the parents — the metagenesis is external. But there is also a kind of metagenesis which we may distinguish as internal. Certain entozoa of the genus Distomaj exhibit it. From the Gg^ of a Dlstoma, there results a rudely- formed creature known to naturalists as the ^' King's-yellow worm.'' Gradually as this increases in size, the greater part of its inner substance is transformed into young animals called Cercarice (which are the larvae of Distomata); until at length, it becomes little more than a living sac, full of living offspring.
In the Dlstoma pacljicay the brood of young animals thus arising by internal gemmation, are not Cercarice, but are of the same form as their parent: themselves becoming the producers of Cercarice after the same manner, at a subsequent period. So that sometimes the succession of forms is represented by the series A, B, A, B, &c.; and sometimes by the series A, B, B, A, B, B, &c. Both cases, however, exemplify internal metagenesis, in contrast with the several kinds of external metagenesis described above. That agamogenesis which is carried on in a reproductive organ — either a true ovarium or the homologue of one — has been called, by Prof. Owen, parthenogenesis. In his work published under this title, he embraced those cases in which the buds arising in the pseud- ovarium, are not ova in the full sense of the word; but rather, as they have since been called by Prof. Huxley, pseud-ova. Yon Siebold and other naturalists, have hence applied the term parthenogenesis to a narrower class of cases. Perhaps it would be best to distinguish this process, which is intermediate between metagenesis and parthenogenesis, by the term jJ5ewc^o-j:'ar^7i<?yio^^;2^S2S. It is the process familiarly exemplified in the Aphides.
Here, from the fertilized eggs laid by perfect females, there grow up imperfect females, in the pseud-ovaria of which there are developed pseud-ova; and these, rapidly assuming the organization of other imperfect females, are born viviparously. From this second generation of imperfect females, there by and by arises, in the same manner, a third generation, of the same kind; and so on for many generations: the series being thus symbolized by the letters A, B, B, B, B, B, &c., A. Respecting this kind of heterogenesis, it should be added, that in animals, as in plants, the number of generations of sexless individuals produced before the re-appearance of sexual ones, is indefinite; both in the sense that in the same species it may go on to a greater or less extent according to circumstances, and in the sense that among the generations of individuals proceeding from the same fertilized germ, a recurrence of sexual individuals takes place earlier in some of the diverging lines of multiplication than in others. In trees we see that on some branches, flower-bearing axes arise while other branches are still producing only leaf-bearing axes; and in the successive generations of Aphides, a parallel truth has been observed. Lastly has to be set down, that form of heterogenesis in which, along with gamogenesis, there occurs a form of agamogenesis exactly like it, save in the absence of fecundation. This is called true parthenogenesis — reproduction carried on by virgin mothers, which are in all respects like other mothers. In the silk-worm-moths this parthenogenesis is exceptional, rather than ordinary: usually the eggs of these insects are fertilized; but if they are not, they are still laid, and some of them produce larvae. In certain LepidojHeirt, however, of the groups Psijchidw and f^KNESIS.
Tinoidw., parthenogenesis appears to be a normal process — indeed, so far as is known, the only process; for of some species the males have never been found.
A general conception of the relations among the different modes of Genesis, thus briefly described, will be best given by the following tabular statement.
{'Oviparous or Genesis is Homogenesis, which is Gamogenesis < Ovo-viviparous or I Viviparous or Heterogenesis, which is < Gamogenesis alternating with ["Parthenogenesis or Agamogenesis < Pseudo-parthenogenesis or r Internal ^ Metagenesis <^ or [ External This, like all other classifications of such phenomena, presents anomalies. It may be justly objected, that the processes here grouped under the head agamogenesis, are the same as those before grouped under the head of discontinuous development (§ 50): thus making development and genesis partially coincident. Doubtless it seems awkward that what are from one point of view considered as structural changes, are from another point of view considered as modes of multiplication.* * Prof. Huxley avoids this difficulty by making every kind of Genesis a mode of development. His classification, which suggested the one given above, is as follows: — Continuous (Growth Metamorphosis Development^ / Metagenesis There is, however, nothing for us but a choice of imperfections. We cannot by any logical dichotomies, accurately express relations which, in Nature, graduate into each other insensibly. Neither the above, nor any other scheme, can do more than give an approximate idea of the truth.
§ 76. Genesis under every form, is a process of negative or positive disintegration; and is thus essentially opposed to that process of integration, which is one element of individual evolution. Negative disintegration occurs in those cases where, as among the compound Hydrozoa, there is a continuous development of new individuals by budding from the bodies of older individuals; and where the older individuals are thus prevented from growing to a greater size, or reaching a higher degree of integration. Positive disintegration occurs in those cases of agamogenesis where the formation of new individuals is discontinuous, and in all cases of gamogenesis. The degrees of disintegration are various. At the one extreme, the parent organism is completely broken up, or dissolved into new individuals; and at the other extreme, the new individual forms but a small deduction from the parent organism. Protozoa and Protojphyta, show us that form of disintegration called spontaneous fission: two or four individuals being produced by the splitting-up of the original one. The Vohox and the Hydrodictyon^ are plants which, having developed broods of young plants within themselves, give them exit by bursting; and among animals, the one lately referred to, which arises from the Distoma ^gg, entirely loses its individuality in the individualities of the numerous Distoma-larYse with which it becomes filled. Speaking generally, the degree of disintegration becomes less marked, as we approach the higher organic forms. Plants of advanced types throw off from themselves, whether by gamogenesis or agamogenesis, parts that are relatively small; and among the higher animals, there is no case in which the parent individuality is habitually lost, in tlio production of new individualities/ To the last, however, there is of necessity a greater or less disintegration. The seeds and pollen-grains of a flowering plant, are disintegrated portions of tissue; as are also the ova and spermatozoa of animals. And whether the fertilized germs carry away from their parents small or large quantities of nutriment, these quantities of nutriment in all cases involve further negative or positive disintegrations of the parents.
'New individuals that result from agamogenesis, usually do not separate from the parent-individuals, until they have undergone considerable development, if not complete development. The agamogenetic offspring of those lowest organisms which develop centrally, do not, of course, pass beyond central structure; but the agamogenetic offspring of organisms that develop axially, commonly assume an axial structure before they become independent. The vegetal kingdom shows us this in the advanced organization of detached bulbils, and of buds that root themselves before separating. Of animals, the Hydrozoa, the Trematoda^ the Salpce, and the ApMdes, present us with different kinds of agamogenesis, in all of which the new individuals are organized to a considerable extent before being cast off. This rule is not without exceptions, however. The winter-eggs of the Plumatella, developed in an unspecialized part of the body, present us with a case of metagenesis, in which centres of development, instead of axes, are detached; and in the above-described parthenogenesis of moths and bees, such centres are detached from an ovarium.
When produced by gamogenesis, the new individuals become independent of the parents while in the shape of centres of development, rather than axes of development; and this even where the reverse is apparently the case. The fertilized germs (3f those inferior plants which are central, or multicentral, in their development, are of course thrown off as centres. In the higher plants, of the two elements that go to the formation of the fertilized germ, the pollen-cell is absolutely » » separated from the parent-plant under the shapa of a centre; and the embryo -cell, though not absolutely separated from the parent, is still no longer subordinate to the organizing forces of the parent. So that when, the embryo-cell having been fertilized by matter from the pollen-tube, the development commences, it proceeds without parental control: the new individual, though remaining physically united with the old individual, becomes structurally and functionally separate while still only a centre of development; and takes on its axial form by processes of its own — the old individual doing no more than supply materials. Throughout the animal kingdom, the new individuals produced by gamogenesis, are obviously separated in the shape of centres of development wherever the reproduction is oviparous: the only conspicuous variation being in the quantity of nutritive matter bequeathed by the parent to the new centre of development, at the time of its separation. And though, where the reproduction is viviparous, the process appears to be different, and in one sense is so; yet, intrinsically, it is the same. For in these cases, the new individual really detaches itself from the parent while still only a centre of development; but instead of being finally cast off in this state, it is re-attached, and supplied with nutriment until it assumes a more or less complete axial structure.
§ 77. Under all its various forms, the essential act in gamogenesis, is the union of two centres or cells, produced by different parent organisms: the sperm-cell being the male product, and the germ-cell the female. There are very many modes and modifications of modes in which these cells are produced; very many modes and modifications of modes by which they are brought into contact; and very many modes and modifications of modes by which th^ resulting fertilized fferms have secured to them the fit conditions for their development. But passing over these many divergent and rc-divergent kinds of sexual multiplication, which it would take too much space here to specify, the one universal peculiarity which it concerns us to remark, is, this coalescence of a detached portion of one organism, with a more or less detached portion of another.
Such protophytes as the PalmellcB and tlie Desjuidiece, which are sometimes distinguished as unicellular plants, show us a coalescence, not of detached portions of two organisms, but of two entire organisms: in the Palmelke, conjugation is a complete fusion of the individuals; and in the Desmidiece, the entire contents of the individuals unite to form the germmass. Where, as among the Confervm, we have aggregated cells whose individualities are scarcely at all subordinate to that of the aggregate, the gamogenetic act is eiFected by the union of the contained granules of two adjacent cells. In Spirogyray it is not adjacent cells in the same thread which thus combine; but cells of one thread with those of another. As we ascend to^ plants of high organization, we find that the two reproductive elements become quite distinct in their characters^ and further, that they arise in different organs set apart for their production: the arrangements being such, that the sperm-cells of one plant combine with the germ-cells of another.
There is reason to think that, among the lowest Protozoa, a fusion of two individualities, analogous to that which occurs in the conjugation of certain Algce, is the process from which results the germ of a new series of individuals. But in animals formed by the aggregation of units that are homologous with Protozoa, the sperm-cells and germ-cells are differentiated. And even in these humble forms, where there is no differentiation of sexes, we have good evidence that, as in all higher forms, the union is not between sperm-cells and germcells that have arisen in the same individual; but between those that have arisen in different individuals.
The marvellous phenomena initiated by the meeting of sperm-cell and germ-cell, naturally suggest the conception of some quite special and peculiar properties possessed by these 220 THE INDITCTIOXS OF BIOLOGY.
cells. It seems obvious that this mysterious power which they disj^lay, of originating a new and complex organism, distinguishes them in the broadest way from portions of organic substance in general. Nevertheless, the more we study the evidence, the more is this assumption shaken — the more are we led towards the conclusion, that these cells have not been made by some unusual elaboration, fundamentally different from all other cells. The first fact which points to this modified conclusion, is the fact recently dwelt upon (§ 63), that in many plants and inferior animals, a small fragment of tissue that is but little difierentiated, is capable of developing into the form of the organism from which it was taken. Conclusive proof obliged us to admit, that the component units of organisms, have inherent powers of arranging themselves into the forms of the organisms to which they belong. And if to these component units, which we distinguished as physiological, such powers must be conceded— if, under fit conditions, and when not much specialized, they manifest such powers in a way as marked as that in which the contents of sperm-cells and germ-cells manifest them; then, it becomes clear that the properties of spermcells and germ-cells are not so peculiar as we are apt to assume. Again, the organs for preparing spermcells and germ-cells, have none of the speciality of structure which might be looked for, did sperm-cells and germcells need endowing with properties essentially unlike those of all other organic agents. On the contrary, these reproductive centres proceed from tissues that are characterized by their low organization. In plants, for example, it is not appendages that have acquired considerable structure, which produce the fructifying particles: these arise at the extremities of the axes, where the degree of structure is the least. The embryo-cells are formed in the undifferentiated part of the cambium-layer; the pollen-grains are formed at the little-differentiated extremities of the stamens; and both are homologous with simple epithelium-cells. Among many inferior animals devoid of special reproductive organs, such as the Hydraf the ova and spermatozoa originate in the layer of indifferent tissue that lies between the endoderm and the ectoderm; that is, they consist of portions of the least specialized substance. And in the higher animals, these same generative agents appear to be merely modified epithelium- cells — cells not remarkable for their complexity of structure, but rather for their simplicity. If, by way of demurrer to this view, it is asked why other epithelium-cells do not exhibit like properties; there are two replies. The first is, that other epithelium- cells are usually so far changed to fit them to their special functions, that they are unfitted for assuming the reproductive function. The second repl}^ is, that in some cases, where the epithelium- cells are but very little specialized, they do exhibit the like properties: not, indeed, by uniting with other epithelium-cells to produce new germs, but by producing new germs without such union. I learn from Dr Hooker, that the Begonia phyllomaniaca habitually develops young plants from the scales of its stem and leaves — nay, that many young plants are developed by a single scale. The epithelium- cells composing one of these scales, swell, here and there, into large globular cells; form chlorophyll in their interiors; shoot out rudimentary axes; and then, by spontaneous constrictions, cut themselves ofi"; drop to the ground; and grow into Begonias. It appears, too, that in a succulent English plant, the Mcdaxis paludosciy a like process occurs: the self-detached cells being, in this case, produced by the surfaces of the leaves. Thus, there is no warrant for the assumption that sperm-cells and germ- cells possess powers fundamentally unlike those of other cells. The inference to which the facts point, is, that they difier from the rest, mainly in not having undergone modifications such as those by which the rest are adapted to particular functions. They are cells that have departed but little from the original and most general type. Or, in the words suggested by a friend, it is not that they are peculiarly specialized, but rather that they are unspecialized: such specializations as some of them exhibit in the shape of locomotive appliances, &c., being interpretable not as intrinsic, but as extrinsic, modifications, that have reference to nothing beyond certain mechanical requirements. Sundry